Project C09 - THz RFID Tags & Components enabled by Additive Manufacturing
Motivation and Objectives
A key scenario and vision of MARIE is that a flying robot will provide an ultimate transparent view about a contaminated or smoked or even burning building or industrial hall to the firefighter to autonomously localize the source of a fire or unconscious people within the burning building, to reliably detect cables and artefacts inside a wall, or, more generally, to systematically create material maps, i.e. to characterize and get information about all relevant materials in the building, also about their status, e.g. spatial temperature data or gas/contamination composition.
To precisely localize the fire source, people, objects and such materials with sub-mm resolution by the flying robot directly in line with MARIE’s in-room contamination/fire scenario application, (1.) terahertz frequencies were chosen because of large bandwidth available, allowing high localization accuracy, and (2.) a passive chipless infrastructure was defined to attain energy-autonomy and robustness, including high-temperature stability by using appropriate materials.
Hence, for precise self-localization of the mobile robot, equipped with an Radio Frequency (RF) transceiver or reader, use is made of fixed localization landmarks in the building or industrial hall, where each is built up of a passive Retro-Reflective Tag (RRT), which provides a unique code in its retro-reflected signal for IDentification after being interrogated by the reader. Since the positions of these landmarks are defined, the reader is able to locate itself by trilateration.
Current radio-based methods for locating mobile devices within buildings are limited by multipath fading, resulting in position errors of several decimeters. Therefore, one of the main objectives of MARIE is to achieve sub-mm accuracy for localization in heavily cluttered and potentially high temperature environments to realize material characterization with mobile devices. On this, the proposed self-localization concept relies on THz chipless RRT landmarks, which requires no power source. Compared to ultrasound and optical systems, the propagation of low THz waves is less affected by air temperature changes or smoke. The necessary passively-generated RRT frequency signatures (code) for IDentification are implemented by specific Frequency Selective Surfaces (FSS), but preferred by the presence or absence of resonators with different resonance frequencies embedded into a Photonic Crystal (PhC) structure. These resonators being excited by higher-order modes with a high loaded quality factor QL inherently show a long ringing time, which additionally allows efficient separation of fast-decaying clutter from the late-time backscattered signal from the tag by appropriate time gating methods.
For the RRT realization in this project, ceramic materials are of particular interest, since their robustness and high temperature stability allow for their use at temperatures above 400°C, which is one prerequisite for an in-room fire scenario. Before the 1st phase of MARIE, research on ceramic chipless RFID and sensors were focused on frequencies below 10 GHz. In the 1st phase, C09 studied the potential and performance of different materials, technologies, and manufacturing techniques for the realization of robust RRTs with high-Q PhC resonators or low-Q FSSs as coding structures, considering readout range, number of distinguishable IDs for a certain bandwidth, temperature stability and performance in cluttered environments.
Achieved Results in the 1st Phase
During the 1st phase of C09, two main paths have been followed towards the realization of the RRTs, facing several challenges which are listed below with their respective solutions.
1 - Fundamental evaluation of suitable materials and technologies, which enable the realization of 80 and 230 GHz tags based on high-Q resonators (coding particles) for IDentification.
· Cylindrical ceramic Dielectric Resonators (DiRs) with low-order modes can be easily manufactured with relative high Q at lower frequencies, but not at THz, because no suitable low-loss materials with high relative permittivities > 20 are available at 230 GHz. Thus, the targeted high-Q factors (> 400) could not be achieved with low-order DiR modes. High-order DiR modes result in higher mode density, and hence, in a very limited frequency range available for the different resonance frequencies of the DiRs, which decreases the number of bits or IDs significantly.
Solution: Photonic Crystal (PhC) structures allow for high-Q factors in low-order modes with low loss materials of relative permittivities around 10, which are available with HR-Si (C09, C12) and alumina (C13) at 230 GHz.
· Any small change in the DiR nearfield shifts the resonance frequency and potentially increases the losses. Moreover, the high-Q resonators need a well-defined structure, holding them in a mechanically-stable position.
Solution: Therefore, high-Q resonators are integrated within a PhC structure of the same low loss material, avoiding additional losses from carrier materials (in particular from metals). The PhC structure keeps the EM fields in specific areas, so that the PhC substrate can be touched by support structures (for mounting/handling) outside of these areas with no effect on the EM performance.
· The tags must work at high temperatures reliably. However, materials such as HR-Si rapidly increase their conductivity, and therefore, their losses with temperatures significantly above 60°C. Hence, realization of high-Q HR-Si resonators above 60°C is not possible.
Solution: In contrast to HR-Si, ceramic materials show excellent temperature stability. However, the fabrication of detailed μm-scale structures is often a challenge, due to their mechanical properties. In close collaboration between C09 and C13, LCM alumina high-Q resonators have been developed and characterized at 80 and 230 GHz (see Fig. 1), showing very promising results with laboratory-confirmed temperature stability up to 120°C  and having the potential to maintain their high-Q up to least 400°C, as demonstrated at 2.5 GHz.
Fig. 1: LCM alumina 4-bit RRT at 230 GHz. a) Four resonators with different resonance frequencies are integrated in the PhC structure, demonstrating that PhC high-Q resonators can be manufactured at the targeted frequencies. b) Frequency response where each bit of information is determined by the presence or absence of a certain resonator, in this case 1111.
2 - In parallel to the activities in 1), the range at which the resonance frequency of single high-Q resonators can be readout has been shown to be very low and unstable with regard to the interrogation angle [4, 7]. For this reason, research has been done on techniques to considerably increase the RCS, and hence, the readout range as described below:
· DiR arrays embedded into foam for mechanical positioning and Frequency Selective Surfaces (FSSs) as illustrated in Fig. 2, have shown an increase in the RCS with respect to a single DiR, as studied in the project S04. However, these configurations are not retroreflective, i.e. the main beam of the backscattered wave is not pointing towards the monostatic reader antenna, but typically away from it.
Solution: As one result of the collaboration between C09 and S04 , it has been demonstrated that the retroreflective issue can be solved by placing a trihedral Corner Reflector (CR) as shown in Fig. 2 behind the DiR array or FSS, so that the transmitted wave is backscattered towards the reader as sketched in Fig. 3. Measurements with up to a 4 m readout range at 80 GHz with a FSS (stopband of bN 5 GHz) in front of a 3x3x3 cm³ trihedral CR have been successfully demonstrated with a RCS of about -7 dBm2 in the IMPb laboratory.
Fig. 2: Low-Q and high-RCS RRTs. a) Conceptual cylindrical DiR array, b) fabricated FSS for frequency coding and 3x3x3 cm3 trihedral CR for an 80 GHz high-RCS RRT. c) Frequency response of the FSS-coded RRT. The ID is encoded in the notch position.
· The feasible Q-factor of the DiR array is inherently limited similar to the FSS by Q < 50 at 230 GHz. This is because, for every interrogation angle, all DiRs are excited, and therefore, contribute to the coding of the backscattered signal. For proper operation, all DiR resonators should resonate exactly at the same resonance frequency. However, this condition becomes increasingly difficult with the targeted higher quality factor and resonance frequency. In combination with the CR, the CR enables the precise localization with a large readout range, while the DiR array and FSS creates one frequency notch with a stopband of bN, enabling the coding with only 1 ID per DiR array or FSS with a frequency separation of Df » bN (see Tab. 1). Additionally, the RCS of the CR has an inherently strong dependency on the interrogation angle.
Solution: As an alternative, a first approach (concept in Fig. 3) for the hybrid integration of a Luneburg lens made of Rogers 5880 with ten high-Q resonators (5 tags x 2 bits) embedded into a PhC structure made of Rogers 6010.2 LM at 80 GHz has been demonstrated at the IMPb (Fig. 4). The distribution of resonators along the PhC plane increases the coverage angle of the RRTs. While the measured maximum readout range of a single tag without lens is approximately only 15 cm, the characterization of the Luneburg lens with high Q resonators has already been performed at 60 cm distance as show in Fig. 4. The measured RCS is -16 dBm2 and the 2 bits (4 IDs) are coded using a bandwidth below 4 GHz. The bandwidth per bit could even be further reduced with higher Q resonators made of alumina or HR Si.
Fig. 3: Comparison between a high-Q and a low-Q RRT. a) considered scenario and b) received reader signal with additional clutter at 230 GHz, where a higher Q factor increases the readout robustness against clutter due to a longer ringing time.
Table 1: Bandwidth and IDs for high-Q Luneburg lens and low-Q trihedral corner reflector.
Low‑Q resonator / FSS
@ 230 GHz
1 GHz (betw. two resonance peaks)
12.5 GHz (betw. two notches)
Coding for 4
400 to 500 (Al2O3)
Hence, the materials and processes used for the hybrid device prove the principal viability of the concept. However, further research is required
· to optimize the lens for an increased gain, readout range and localization accuracy
· to achieve higher Q factors for a longer ringing response to suppress clutter efficiently
· to increment the number of resonators (bits) from 2 to 4 or 8 for increasing the number of IDs º 2bits
· and to reduce the RCS variation for localization over a wide-angle scan.
The RCS of the retroreflective configuration needed for a certain readout range Rmax at 230 GHz can be estimated from a power link budget similar to Fig. 4. Here the assumptions for the radiated power Ps=0 dBm and the receiver sensitivity P(r min) = -80 dBm. The graphs are plotted for a reader antenna gain of 25 dBi and 35 dBi, respectively. For both, the trihedral corner reflector and the Luneburg lens, the analytical RCS are calculated and plotted for normal incidence and a ±40° angle scan. The Luneburg lens demonstrates a clear improvement in RCS angle dependence. However, for future applications, a received power above P(r min) is desired, as the long ringing response must outlast the reflections from nearby objects (about 25 dB, see Fig. 3).
Fig. 4: Maximum readout range Rmax at 230 GHz for different RCSs and reader antenna gains G. The horizontal lines in blue and orange represent the angular RCS variation for normal and 40° incidence.
In the 2nd phase of MARIE C09 will be merged with C13.
Selected Project-related publications
For all project-related publications please click here and scroll to the C09 section.
 A. Jiménez-Sáez, M. Schüßler, C. Krause, F. Meyer, G. vom Bögel and R. Jakoby, "Photonic crystal thz high-q resonator for chipless wireless identification", in 2018 First International Workshop on Mobile Terahertz Systems (IWMTS), Germany, 2018.
 A. Jiménez-Sáez, M. Schüßler, C. Krause, D. Pandel, K. Rezer, G. vom Bögel, N. Benson and R. Jakoby, "3D printed alumina for low-loss millimeter wave components", IEEE Access 7, Apr. 2019.
 Y. Zhao, J. Weidenmueller, G. vom Bögel, A. Grabmaier, A. Alhaj Abbas, K. Solbach, A. Jiménez-Sáez, M. Schüßler and R. Jakoby, "2D Metamaterial Luneburg Lens for Enhancing the RCS of Chipless Dielectric Resonator Tags", in 2019 Second International Workshop on Mobile Terahertz Systems (IWMTS), Germany, Jul. 2020.
 A. Jiménez-Sáez, M. Schüßler, D. Pandel, N. Benson and R. Jakoby, "3D Printed 90 GHz Frequency-Coded Chipless Wireless RFID Tag", in 2019 IEEE MTT-S International Microwave Workshop Series on Advanced Materials and Processes for RF and THz Applications (IMWS-AMP), Germany, Jul. 2020.
 A. Jiménez-Sáez, M. Schüßler, M. El-Absi, A. Alhaj Abbas, K. Solbach, T. Kaiser and R. Jakoby, "Frequency Selective Surface Coded Retroreflectors for Chipless Indoor Localization Tag Landmarks", IEEE Antennas and Wireless Propagation Letters, Mar. 2020.
 A. Jiménez-Sáez, M. Schüßler, D. Pandel, C. Krause, Y. Zhao, G. vom Bögel, N. Benson and R. Jakoby, "Temperature Characterization of High-Q Resonators of Different Materials for mm-Wave Indoor Localization Tag Landmarks", 2020 14th European Conference on Antennas and Propagation (EUCAP), Denmark, Mar. 2020, waiting for publication.
 A. Jiménez-Sáez, A. Alhaj-Abbas, M. Schüßler, A. Abuel-Haija, M. El-Absi, M. Sakaki, L. Samfaß, N. Benson, M. Hoffmann, R. Jakoby, T. Kaiser and K. Solbach, "Frequency-Coded mm-Wave Tags for Self-Localization System Using Dielectric Resonators", Journal of Infrared, Millimeter, and Terahertz Waves, https://doi.org/10.1007/s10762-020-00707-0.